Heat oven to 400°. Place almonds in a food processor; process until finely ground. Add flour, 1/4 cup of the sugar, the butter, egg yolk and salt; pulse until butter forms coarse crumbs. (If it seems too dry, add 1 tbsp cold water.) Press mixture into a 9-inch tart pan with a removable bottom.
Toss plums with remaining 1/4 cup brown sugar and the lemon zest. Arrange in the tart pan in 3 concentric circles, alternating the direction of the slices with each circle. Bake at 400° for 40 minutes on the middle rack, until crust is lightly browned and plums are tender.
In a small bowl, combine apricot preserves with 1 tbsp. water; brush on plums. Cool completely on a wire rack.
